Emails from the new server would be delivered to the old one, but any response from the old one would result in the following message:
Your message wasn't delivered because of security policies. Microsoft Exchange will not try to redeliver this message for you. Please provide the following diagnostic text to your system administrator.
After much investigation (and a costly call to Microsoft) we found that the permissions on the Default Receive Connector was incorrect and I needed to grant Authenticated Users the "Accept Authoritative Domain Send" permission.
Once this was applied, emails flowed perfectly :)
Hopefully this hint helps you out, please click on the adverts on this page to show your appreciation.
